An adjustable diffusion-based microfluidic reactor is presented here, which is based on electro-osmotic guiding of reagent samples. The device consists of a laminar flow chamber with two separate reagent inlets. The position and the width of the two sample streams in the flow chamber can be controlled individually by changing the flow ratio of three parallel guiding buffer streams. Since electro-osmotic flow (EOF) is used for pumping, no external pumps or other moving parts are needed. The region where the diffusive profiles of the two sample streams overlap is used for the reactions. This overlapping region can be manipulated in a predictable way by adjusting the voltages required to generate the respective electro-osmotic flow. Reaction dynamics inside the microreactor is illustrated with a reactant pair of a fluorescent calcium tracer and a calcium chloride solution. An analytical model, which is an analogue of electrical circuits to EOF, was developed and embedded into the LabView control software, allowing real-time control of the microreactor. This paper describes the simulation, fabrication and experimental characterisation of the device.